The present invention generally relates to magnetic sensors and more particularly to a Hall-effect magnetic sensor and a magnetic head that uses such a magnetic sensor.
Magnetic heads are used extensively from audio-visual apparatuses such as video tape recorders and audio tape recorders to information processing apparatuses such as a personal computer. In the computers for use in the so-called multimedia applications, in particular, there is a ponderous demand for recording an enormous amount of information such as image data and audio data. Thus, in order to realize such a high speed, large capacity magnetic storage device, it is necessary to develop a high sensitivity and high resolution magnetic head that performs reading and writing with a correspondingly high resolution.
FIGS.1A-1D show the construction of a conventional thin-film magnetic head 1 used in a conventional hard disk drive.
Referring to FIG. 1A, the magnetic head 1 is formed on a rear edge surface 2a of a carrier piece 2 that floats away from a recording surface of a revolving magnetic disk by an air foil. In order to create the necessary air foil, the carrier piece 2 is formed with a bearing surface 2b that forms a fluid bearing.
As indicated in FIG. 1B, a coil pattern 1a is formed on the foregoing rear edge surface 2a together with interconnection pads 1b and 1c provided at both ends of the coil pattern 1a. Further, a magnetic yoke 1d is formed also on the rear edge surface 2a. It should be noted that the magnetic yoke 1d forms a magnetic gap 1g such that the magnetic gap 1g is exposed at bearing surface 2b.
FIG. 1C shows the magnetic head 1 in a cross sectional view taken along a line that intersects the foregoing magnetic yoke 1d. It will be noted that the magnetic yoke 1d includes a lower pole piece 1d.sub.1 provided on the foregoing edge surface 2a and an upper pole piece 1d.sub.2 provided above the lower pole piece 1d.sub.1, wherein the foregoing coil pattern 1a is formed in a space defined between the upper pole piece 1d.sub.2 and the lower pole piece 1d.sub.1. The upper and lower pole pieces 1d.sub.1 and 1d.sub.2 are connected with each other at respective rear ends to form a closed magnetic circuit that passes through the magnetic gap 1g. It should be noted that the magnetic gap 1g is formed between respective front ends of the upper and lower pole pieces 1d.sub.1 and 1d.sub.2.
FIG. 1D shows the details of the magnetic gap 1g in an enlarged scale. Referring to FIG. 1D, it will be noted that the pole pieces 1d.sub.1 and 1d.sub.2 come closer with each other at respective front ends thereof to form a gap having a minute gap width of 1 .mu.m or less.
In such a conventional magnetic head, the maximum recording density that the magnetic head can attend to, in other words the resolution of the magnetic head, is determined by the gap width of the magnetic head and the distance from the magnetic head to a magnetic recording medium. In the conventional magnetic head that uses a coil wound around a magnetic core, a recording density of about 65 Mbits/inch.sup.2 is already achieved by using a gap of 1 .mu.m. However, it is expected that, in future, a super-high-density magnetic head that can successfully perform reading and writing with a recording density exceeding 20 Gbits/inch.sup.2, becomes necessary.
In order to realize such a super-high-resolution resolution magnetic head, it is necessary to provide a magnetic sensor having a super-high-sensitivity that can pick up an extremely feeble magnetic signal. It should be noted that, in the recording density of 20 Gbits/inch.sup.2, each bit of the information signal forms a magnetization spot on a magnetic recording medium with a size of about 45 nm.times.750 nm. Thus, the magnetic head for such a super-high-density recording should have a gap width close to 45 nm. As long as a conventional electro-magnetic conversion device is used, the magnetic head cannot detect such an extremely minute magnetization spot at high speed. In other words, the conventional magnetic head cannot provide necessary sensitivity and response.
As a super-high-sensitivity magnetic head capable of detecting such an extremely small recording dot, it is proposed to use a magnetic head that is equipped with a magnetoresistive sensor. See, for example, P. Ciureanu and H. Gavrila, Studies in Electrical and Electronic Engineering 39, "Magnetic Heads for Digital Recording," Chapter 7, Elsevier Publication, 1990.
In such a magnetic head equipped with a magnetoresistive sensor, detection of a recording dot is made by detecting a change in the resistance of a ferromagnetic film, typically a film of a polycrystalline Fe--Ni alloy. It should be noted that such a polycrystalline ferromagnetic film shows a uniaxial magnetic anisotropy defined by an easy axis of magnetization and a hard axis of magnetization, and changes the direction of magnetization with respect to the foregoing two magnetic axes, in each of magnetic domains formed in the film, in response to an external magnetic field. As the resistance of the ferromagnetic film changes in response to the direction of magnetization with respect to the magnetic axes, it is possible to detect the external magnetic field, and hence the magnetic recording bit, by measuring the resistance of the ferromagnetic film. It should be noted that the resistance of the ferromagnetic film becomes maximum when the direction of magnetization is parallel to the easy axis of magnetization and becomes minimum when the direction of magnetization is perpendicular to the easy axis of magnetization.
Meanwhile, it is preferable, in a general magnetic head including also the super-high-sensitivity magnetic head, to carry out reading and writing by using the same, single magnetic gap for avoiding off-track loss. Thus, it is more preferable to construct a single gap head that includes only one magnetic gap as compared with a double gap head that includes two magnetic gaps one for recording and one for reading.
In this respect, the magnetoresistive sensor using the ferromagnetic film is advantageous for constructing a reading head inside a writing head because of the reduced size of the sensor. On the other hand, such a construction of providing a magnetoresistive sensor inside the gap of the magnetic writing head causes a problem in that the strong magnetic field formed in the magnetic gap at the time of writing may induce a rearrangement in the magnetic domains in the ferromagnetic film. It should be noted that the magnetic field inside the magnetic gap can exceed 2000 Gauss. Thereby, the magnetoresistive characteristics of the ferromagnetic film may be changed as a result of the writing, and the reading by using such a magnetoresistive sensor becomes inevitably unstable.
As a magnetic sensor immune to the magnetic field acting at the time of writing, it is proposed to use a semiconductor magnetic sensor that utilizes the Hall effect, for the magnetic head. In such a semiconductor magnetic sensor, a two-dimensional electron gas is formed at a heterojunction interface of semiconductor layers forming a modulation doped structure, and the detection of the magnetic field is achieved by detecting the Hall effect induced in such a two-dimensional electron gas. As the two-dimensional electron gas exhibits an extremely large electron mobility, the semiconductor magnetic sensor shows a correspondingly high sensitivity of magnetic detection.
FIG. 2A shows an example of such a Hall-effect semiconductor magnetic sensor while FIG. 2B shows a part of the magnetic sensor in detail.
Referring to FIGS.2A and 2B, the semiconductor magnetic sensor is constructed upon a semiconductor layered body 21 to be described later in detail with reference to FIG. 3, and includes electrodes 22A and 22B disposed on the upper major surface of the layered body 21 in alignment in a first direction to form a first electrode pair, and electrodes 22C and 22D disposed on the upper major surface of the layered body 21 in alignment in a second direction perpendicular to the first direction, to form a second electrode pair. In the illustrated example, the electrode 22A acts as a source electrode and injects electrons into the semiconductor layered body 21, while the electrode 22B acts as a drain electrode and collects the electrons injected by the electrode 22A and passed through the semiconductor layered body 21. Further, the Hall-effect is detected by measuring the Hall-voltage appearing across the electrodes 22C and 22D.
It should be noted that the semiconductor layered body 21 has a modulation doped structure similar to that used in a HEMT, and includes an undoped active layer 21.sub.-1 formed on a semi-insulating substrate, an n-type electron supplying layer 21.sub.-3 provided on the active layer 21.sub.-1, with a thin spacer layer 21.sub.-2 intervening between the active layer 21.sub.-1 and the electron supplying layer 21.sub.-3, for supplying electrons into the active layer 21.sub.-1, and an n-type contact layer 21.sub.-4 formed on the electron supplying layer 21.sub.-3, wherein the foregoing electrodes 22A-22D are formed on the upper major surface of the contact layer 21.sub.-4. Similar to ordinary HEMTs, a two-dimensional electron gas (2DEG) is formed in the active layer 21.sub.-1 along a heterojunction interface to the spacer layer 21.sub.-2.
FIG. 3 shows the modulation doped structure of the semiconductor layered body 21 in more detail.
Referring to FIG. 3, the semiconductor layered body 21 is formed on a semi-insulating GaAs substrate 210, and includes an undoped buffer layer 211 of GaAs formed on the substrate 210 with a thickness of about 500 nm, wherein the buffer layer 211 carries thereon a first superlattice structure formed of an alternate stacking of an undoped AlAs layer 212 having a thickness of 1.5 nm and an undoped GaAs layer 213 having a thickness of 2.5 nm. It should be noted that the first superlattice structure includes the layers 212 and 213 repeated n times, and corresponds to the active layer 21.sub.-1 of FIG. 2B.
On the first superlattice structure, there is provided a second superlattice structure comprising a stacking of an undoped layer 214 of AlAs having a thickness of 1.5 nm, an undoped layer 215 of GaAs having a thickness of 0.5 nm, an n-type layer 216 of GaAs having a thickness of 1.5 nm, and an undoped layer 217 of GaAs having a thickness of 0.5 nm, wherein the layer 214 contacts with the uppermost layer of the first superlattice structure at the lowermost part of the second superlattice structure as a layer corresponding to the spacer layer 21.sub.-2. Further, the foregoing layers 214-217 are repeated m times to form the second superlattice structure, wherein the second superlattice structure corresponds to the electron supplying layer 21.sub.-3 of FIG. 2B. Further, an n-type layer 218 of GaAs is provided on the second superlattice structure with a thickness of 10 nm in correspondence to the contact layer 21.sub.-4 of FIG. 2B. It should be noted that the layer 218 carries thereon the foregoing electrodes 22A-22D.
In such a modulation doped structure, it should be noted that the electrons supplied from the n-type GaAs layer 216 in the second superlattice structure form a two-dimensional gas (2DEG) in the undoped GaAs layer 211 along the interface to the spacer layer 212, and the two-dimensional electron gas thus formed causes a Hall-effect upon application of an external magnetic field as already noted. By employing the superlattice structure, it is possible to increase the electron density of the two-dimensional electron gas.
Generally, a Hall-effect magnetic sensor provides a Hall voltage V.sub.H such that the Hall voltage is proportional to the mobility .mu. of the carrier in a conductive layer to which the magnetic field is applied and is inversely proportional to the thickness d of the conductive layer (V.sub.H .infin..mu./d), wherein the conductive layer is provided, in the case of the foregoing semiconductor structure, by the two-dimensional electron gas. As the electron mobility .mu. in such a two-dimensional electron gas is enormous, and as the thickness d of the two-dimensional electron gas is minimum, the semiconductor magnetic sensor that uses such a two-dimensional electron gas provides a very large sensitivity.
When using such a Hall-effect magnetic sensor in a thin-film magnetic head as shown in FIGS.1A-1D, it is necessary and desirable to accommodate the magnetic sensor in the gap 1g formed at the tip end of the magnetic yoke. For this to be achieved, it is necessary to reduce the overall thickness of the Hall-effect magnetic sensor. This is particularly important in the super-high-sensitivity magnetic head used for super-high-density recording. In the Hall-effect magnetic sensor that uses the normal modulation doped-structure shown in FIG. 3, the semiconductor layered body formed on the buffer layer 211 generally has a thickness of about 500 nm, while the buffer layer 211 itself has a thickness of about 500 nm. This means that, even if the semi-insulating substrate 210 is removed entirely, the total thickness of the magnetic sensor still exceeds 1000 nm.
FIG. 4 shows the scanning of a recording mark A by means of a thin-film magnetic head having a gap width of 500 nm, along a recording track of which width is set to 750 nm. In such a case, the recording density achieved on the recording medium is about 1 Gbits/inch.sup.2. It should be noted that the area of the recording mark A is 3.75.times.10.sup.-9 cm.sup.2, while this value corresponds to the recording density of 1/5.8.times.10.sup.-10 bits/inch.sup.2. The latter is approximately equal to the recording density of 1 Gbits/inch.sup.2.
The evaluation of FIG. 4 indicates that, in order to achieve a recording density reaching the expected value of 20 Gbits/inch.sup.2, it is necessary to reduce the gap width of the thin film magnetic head to be about 50 nm or less. For this purpose, it is necessary to reduce the thickness of the Hall-effect magnetic sensor to be approximately 50 nm or less.
FIG. 5 shows a diagram explaining the problem that arises when the thickness d of the semiconductor layered body formed on the substrate 210 of. FIG. 3 is reduced. In FIG. 5, the semiconductor layers 212-218 formed on the active layer 211 are collectively designated by a reference numeral 221. Further, the total thickness of the semiconductor layered body 221 thus defined is designated by S.
Referring to FIG. 5, it will be noted that there is formed a surface depletion region indicated by hatching in such a semiconductor structure, such that the surface depletion region extends from the surface of the semiconductor layered body 221 toward the substrate 210. Thus, when the thickness S of the semiconductor layered body 221 is reduced, the surface depletion region inevitably comes closer to the two-dimensional electron gas (2DEG) formed in the active layer 211. With a further reduction in the thickness S, the surface depletion region ultimately reaches the two-dimensional electron gas itself.
When the surface depletion region penetrates deeply into the semiconductor layered body 221 that includes the electron supplying layer 216 or reaches the two-dimensional electron gas itself, the two-dimensional gas, which is essential for the operation of the Hall-effect sensor, vanishes. Thereby, the Hall-effect no longer occurs and the device does not operate as a Hall-effect magnetic sensor. A similar problem occurs also when the substrate 210 is removed and the thickness of the layer 211 is reduced.
Thus, it will be noted that, while the Hall-effect magnetic sensor having a conventional modulation doped structure may have an advantageous feature of extremely high sensitivity and extremely high resolution, there is a serious problem in that the two-dimensional electron gas vanishes when the thickness of the device is reduced so as to enable accommodation of the Hall-effect semiconductor magnetic sensor in a magnetic gap of a high resolution thin-film magnetic head.